When a hydrogenation process is applied to an aromatic nitro compound carrying halogen atoms bonded to the aromatic nucleus, the conversion of the nitro group to an amino group is invariably followed by hydrogenolysis of the carbon-halogen bond to give, on the one hand, the dehalogenated nucleus and, on the other hand, hydrogen halide acids. This phenomenon has been known for a very long time, since it was described by P. Sabatier and A. Mailhe in 1904.
Many studies have been conducted to avoid this secondary reaction while allowing the catalyst to remain properly active. These studies have led to various solutions which can be classified into two groups: those employing platinum or palladium and those employing Raney nickel as the hydrogenation catalyst. All these studies involve the use of a modified catalyst.
In the first group of techniques, those employing metals of the platinum group, are processes in which the hydrogenation catalyst employed is platinum deposited on carbon, optionally inhibited by the presence of an adjuvant referred to by the neologism "selectivity agent", such as thioethers and disulfides. Although the degree of dehalogenation is very low, this technique still presents numerous disadvantages, including the formation of highly toxic byproducts such as diazo derivatives, and the very high cost of the catalyst.
In the second group of techniques, those which employ the metals of the first row of group VIII and especially nickel in the form of Raney nickel, the manufacture of diazo derivatives is avoided, but the formation of other interfering products remains very high, especially the products resulting from hydrodehalogenation (hydrogenolysis of carbon-halogen bonds). Hydrogenolysis products are particularly troublesome in the manufacture of fluorinated anilines as it is not possible to separate fluorinated anilines from unsubstituted anilines at costs which are reasonable relative to the selling price of the said fluorinated anilines.
Thus, attempts have been made to make the hydrogenation reaction more selective by employing selective catalyst poisons, poisons which are referred to in this context as selectivity agents.
Thus, the use of Raney nickel to which a calcium or magnesium hydroxide is added as a selectivity agent has been recommended. In order to avoid dehalogenation, the reaction temperatures must be very moderate, and this does not allow these processes to be employed on an industrial scale.
It has also been proposed to employ Raney nickel in combination with thiocyanate, alkylamine, alkanolamine, a heterocyclic base, trialkyl phosphite, cyanamide or dicyandiamide. None of these improvements has neatly solved the problem, especially since it is easier to use a material which intrinsically offers the required qualities as a catalyst rather than a catalyst which has to be poisoned with sufficient adroitness so that it will catalyze only certain reactions and will do so, moreover, without losing most of its catalyst effectiveness in these reactions.
These difficulties are also encountered during the synthesis of anilides, especially of acylated amino compounds from the corresponding nitro derivatives (or, in a broader sense, in the synthesis of intermediates between the nitro derivatives and the anilines, whether identified or not).
In fact, during conventional syntheses of anilides it is often necessary to carry out the reduction and the amidation in several stages and it is necessary to employ powerful reactants such as anhydrides, or mixed anhydrides.